Currently, ultrasonic imaging devices are used to diagnose body tissue surrounding a body cavity, such as, e.g., the wall of a blood vessel. One type of an ultrasonic imaging device includes an invasive catheter tube with an internally disposed rotatable imaging core, the imaging core including a drive cable having a distally mounted transducer that is situated in an acoustic imaging window provided at a distal end of the catheter tube. The transducer is electrically coupled to a signal generator/processor via signal wires disposed in the drive cable. To further facilitate transmission of ultrasonic energy, the imaging window of the catheter tube is filled with an aqueous solution, thereby immersing the distally mounted ultrasonic transducer. An example of such a device is disclosed in U.S. Pat. No. 5,000,185.
When the distal end of the ultrasonic imaging catheter, and thus the ultrasonic transducer, is placed adjacent the body tissue to be diagnosed, the signal generator/processor transmits an electrical output signal via the signal wires electrically stimulating the ultrasonic transducer. As a result, the ultrasonic transducer emits ultrasonic energy, which is transmitted through the aqueous solution filled acoustic window and into the surrounding tissue. As the ultrasonic transducer is electrically stimulated, the drive cable is rotated at a high speed, thereby rotating the attached ultrasonic transducer and causing the ultrasonic energy to be transmitted into a 360.degree. circumferential portion of the surrounding tissue.
A portion of the ultrasonic energy is reflected from the surrounding tissue back to the ultrasonic transducer, stimulating the ultrasonic transducer to produce a response electrical signal, which is transmitted back to the signal generator/processor via the signal wires. The response signal is interpreted by the signal processor and translated into a 360.degree. image slice of the surrounding tissue, which can be seen by an attending physician on a monitor. From this image, the composition and physical characteristics of the surrounding tissue can be determined.
As depicted in FIG. 1, a typical ultrasonic transducer 10 includes a piezoelectric element 12, which is the active element that emits ultrasonic energy in response to electrical stimulation originating from a signal generator (not shown). The ultrasonic transducer 10 further includes a matching layer 14 and a backing layer 16, which are formed on opposite sides of the piezoelectric element 12. The matching layer 14 allows passage of ultrasonic energy therethrough. The backing layer 16 reflects and attenuates the ultrasonic energy, thereby facilitating the focussed and unidirectional emission of the energy 22 from the transducer 10 perpendicular to the surface of the matching layer 14. The ultrasonic energy 22 is then transmitted through an aqueous solution 20 in which the ultrasonic transducer 10 is immersed, through an acoustic window (not shown), and into the surrounding tissue (also not shown). The ultrasonic transducer 10 further includes first and second electrodes 18 and 19 formed of electrically conductive material consisting of gold, chrome, nickel or a combination thereof (thickness exaggerated in FIG. 1) for purposes of facilitating electrical connection between the piezoelectric element 12 and a positive, or power signal wire 32, and a negative, or ground signal wire (shown in FIG. 2).
Referring to FIGS. 2 and 3, a known imaging core 24 includes a drive cable 26 with a distally mounted transducer assembly 28. The transducer assembly 28 includes an electrically conductive transducer housing 30 in which the transducer 10 is mounted. The ultrasonic transducer 10 is electrically coupled to a signal generator (not shown) via the power signal wire 32 and the ground signal wire (not shown).
Using a presently known technique, the transducer 10 is mounted in the transducer housing 30, such that the backing layer 16 is in electrical contact with the transducer housing 30, and the matching layer 14 is electrically isolated from the transducer housing 28. The transducer 10 is then fixably bonded to the transducer housing 30. The ground signal wire is welded to drive cable 26 (connection not shown), which is in electrical contact with the transducer housing 30, thereby providing electrical contact with the backing layer 16.
To electrically connect the power signal wire 32 to the ultrasonic transducer 10, a portion 36 of the matching layer 14 (shown in phantom in FIG. 2) is removed, such as, e.g., by etching, to expose the surface of the first electrode 18. The power signal wire 32 is then connected to the exposed surface of the first electrode 18. Electrically conductive material 38 is then formed over the exposed surface of the first electrode 18 to ensure an integral connection between the power signal wire 32 and the first electrode 18. The ultrasonic transducer 10 and the respective power signal wire 32 and ground signal wire (not shown) 32 are encapsulated with a non-conductive material to form a non-conductive potting 40, which ensures electrical isolation between the power signal wire 32 and the ground signal wire.
Although such an ultrasonic transducer mounting and electrical connection technique generally results in an imaging core of relatively high quality, as seen in FIG. 4, disposition of the power signal wire 32 above the piezoelectric element 12 creates a portion 42 on the aperture 44 (i.e., surface area) of the transducer 10, resulting in a disturbance in the distribution of energy 46 over the aperture 44, which reduces the performance characteristics of the transducer 10. Further, the effect of the disturbance varies from one ultrasonic transducer to the next, resulting in inconsistent performance characteristics among identically rated transducers.
In addition, although the matching layer 14 of the transducer 10 is manufactured to a calculated thickness to provide an ultrasonic transducer with theoretically optimized performance characteristics (typically one-quarter wavelength with respect to a specific operating frequency), manufacturing tolerances result in transducers having performance characteristics that vary from these calculated performance characteristics.
Thus, it would be desirable to provide an electrical signal connection to the ultrasonic transducers 10 in a manner resulting in more consistent and optimized performance characteristics.